This invention is directed to a novel method and apparatus for acquiring the in-phase and quadrature components of a bandpass signal.
In digital signal processing systems, it is necessary to extract information from a bandpass signal x(t) of center frequency f.sub.c and bandwidth 2w by the implementation of a sampling process. This sampling process must acquire both the in-phase (I) and quadrature (Q) components of x(t). The bandpass signal x(t) may be represented mathematically as EQU x(t)=x.sub.c (t) cos (2.pi.f.sub.c t)-x.sub.s (t) sin (2.pi.f.sub.c t) (1)
where x.sub.c (t) and x.sub.s (t) are baseband (low pass) signals. The sampling process must acquire from x(t) sampled versions of both x.sub.c (t) and x.sub.s (t).
In the conventional method and apparatus for obtaining the samples of x.sub.c (t) and x.sub.s (t) the signal x(t) is fed into two separate mixers whose local oscillator feeds are 90.degree. apart in phase. After passing the mixer output signals through low pass filters and samplers, the signals so obtained are sampled versions of x.sub.c (t), and x.sub.s (t), namely kx.sub.c (nT) and kx.sub.s (nT), where k is the amplifier gain constant and T is the sampling period.
This method requires two near identical channels with matching mixers, filters and amplifiers. This hardware requirement can be both costly and sometimes difficult to implement. The method also suffers from the fact that since x.sub.c (t) and x.sub.s (t) are both baseband signals, any amplification of x.sub.c (t) or x.sub.s (t) which may be required before sampling must be accomplished with DC amplifiers, which are much more cumbersome and expensive to design than AC amplifiers. An example of this type of system is described in U.S. Pat. No. 3,705,360, which issued to G. Rabow on Dec. 5, 1972.